U.S. Pat. No. 3,673,049 discloses the production of laminated glass articles wherein each lamina exhibits a state of stress which is opposite to the state of stress demonstrated by the contiguous layer(s). Stated differently, the articles described consist of layers manifesting tensile and compressive stress in alternating relationship with one another. The laminated articles are prepared by bringing together streams of molten glass of differing compositions and viscosities such that the laminae are essentially simultaneously laid up in the desired structural order. As is explained therein, where a three-layer laminate is fabricated consisting of a relatively thick interior or core portion surrounded with a thin surface or skin layer, the skin layer displaying a lower coefficient of thermal expansion than the core portion, the surface laminae will be under compression and the interior portion under tension. The surface compression layer imparts greatly enhanced overall mechanical strength to the resultant body.
Because three-ply laminates can be made of very thin cross section but with high mechanical strength, such products have been utilized as tableware where the skin and/or core portion is an opal glass. For example, tableware marketed by Corning Glass Works, Corning, New York, under the trademark CORELLE.RTM., consists of a three-ply laminate wherein the core portion is a spontaneous opal and the skin glass is clear, the skin glass having a lower coefficient of thermal expansion than the opal interior portion.
U.S. Pat. No. 3,673,049 discloses a group of spontaneous opal glasses stated to be especially suitable for use in a three-ply laminate, those glasses consisting essentially, by weight on the oxide basis, of
______________________________________ SiO.sub.2 50-75 Al.sub.2 O.sub.3 3-20 Alkali Metal Oxide 3-20 Alkaline Earth Metal Oxide 0-20 B.sub.2 O.sub.3 + CeO.sub.2 + Bi.sub.2 O.sub.3 + PbO + GeO.sub.2 + CdO + ZnO + Ta.sub.2 O.sub.5 + ZrO.sub.2 + TiO.sub.2 + La.sub.2 O.sub.3 0-10 As.sub.2 O.sub.3 + Sb.sub.2 O.sub.3 0-2 Cl 0-1.5 Nio + V.sub.2 O.sub.5 + Nd.sub.2 O.sub.3 + CuO + CoO + Fe.sub.2 O.sub.3 + MnO.sub.2 + Cr.sub.2 O.sub.3 0-5 F 3-8 ______________________________________
U.S. Pat. No. 3,737,294 is also directed to the hot forming of laminated glass articles by bringing streams of molten glass together in general accordance with the method described above in U.S. Pat. No. 3,673,049, the difference in method involving the viscosities of the streams of glasses being brought together. The patent cites the same group of spontaneous opal glass compositions as being particularly useful for core portions in three-ply laminates as is described in U.S. Pat. No. 3,673,049.
U.S. Pat. No. 3,649,440 discloses the thermal tempering of multi-ply laminated glass articles, hot formed from streams of molten glass, wherein the laminae are alternately in compression and tension, to significantly improve the impact resistance of the articles and inhibit spontaneous breakage arising via bruise checks. The patent provide examples of three-ply laminates wherein the interior portion is an opal glass. No glass compositions are claimed per se but ranges developed from the specific spontaneous opal glasses tabulated in the patent consisted, by weight on the oxide basis, of
______________________________________ SiO.sub.2 64-66.4 Al.sub.2 O.sub.3 6.2-6.3 B.sub.2 O.sub.3 1.3-4.5 MgO 0.7-0.9 CaO 13.6-15.7 Na.sub.2 O 2.1-4.4 K.sub.2 O 1.5-4.1 F 2.7-4.4 ______________________________________
U.S. Pat. No. 3,661,601 is drawn to compositions especially suitable as spontaneous opal glass core portions for three-ply laminates. The base glasses therefor consist essentially, as expressed in weight percent on the oxide basis, of 50-75% SiO.sub.2, 3-9% Al.sub.2 O.sub.3, 11-20% CaO, 1-7% B.sub.2 O.sub.3, and 3-10% Na.sub.2 O+K.sub.2 O consisting of 0-7% Na.sub.2 O and 0-7% K.sub.2 O. The patent notes that MgO may be present as an optional ingredient up to 3%, with the caveat that its presence can reduce the opacity of the glass. The MgO content in the sole working example recorded in the patent containing MgO was 0.7%.
A glass commercially viable as a spontaneous opal core glass in a three-ply glass laminate suitable for tableware will be the result of an appropriate compromise of melting, forming, physical, and chemical properties. Hence, the glass must exhibit the necessary viscosity and liquidus parameters to permit the bringing of a stream thereof into contact with a stream of the skin glass. The glass should demonstrate good resistance to devitrification in the presence of platinum and refractory ceramic forming members and melting unit structures. The glass is required to opalize spontaneously to a body which is densely opaque even in thin cross section. Finally, the core glass must remain relatively stable when subjected to subsequent heat treatments encountered during the application of decorations thereto.
A typical analysis of the core glass utilized in the CORELLE.RTM. brand tableware ranges about:
______________________________________ Al.sub.2 O.sub.3 6.25 .+-. 0.1 Na.sub.2 O 3.05 .+-. 0.1 K.sub.2 O 2.95 .+-. 0.1 MgO 0.7 .+-. 0.1 B.sub.2 O.sub.3 4.7 .+-. 0.1 F 3.45 .+-. 0.1 CaO 15.0 .+-. 0.15 Fe.sub.2 O.sub.3 0.06 .+-. 0.03 SiO.sub.2 64.0 .+-. 1.0 ______________________________________
The MgO and Fe.sub.2 O.sub.3 contents are not added as specific components but find their way into the glasses as impurities in the raw batch materials or cullet utilized in the melting process.
As is explained in U.S. Pat. No. 3,661,601, supra, as the molten glasses of that disclosure are cooled, an amorphous opacifying phase is formed which consists of phase separated droplets. Those droplets are believed to contain CaO and F or CaO, F, B.sub.2 O.sub.3, and SiO.sub.2 with X-ray diffraction analyses of the droplets showing that the droplets are non-crystalline. However, upon heat treatment at temperatures above the annealing point of the glass, the droplets become crystallized, the extent of crystallization being dependent upon the time and temperature of the heat treatment.
As has been discussed above, the high mechanical strength exhibited by the three-ply laminates results from the differences in thermal expansion existing between the core and skin glasses, the skin glass having a lower coefficient of thermal expansion than the core glass so as to develop a surface compression layer on the body. However, this difference in thermal expansion cannot be of such magnitude as to lead to the development of excessively high central tension. Rather, the force exhibited upon ware breakage should be relatively mild, resulting in but a few large pieces. Hence, the respective compositions of the skin and core glasses must be carefully monitored.
In the past, CORELLE.RTM. brand tableware had been decorated with various glazes and enamels which required about a five-minute heat treatment at temperatures above the annealing point, but below the softening point of the ware. However, when different decorations were desired which involved substantially longer heat treatments at similar temperatures, e.g., 15 minutes, the mechanical strength of the laminated product sharply decreased.
Examination of the core glass and skin glass after that latter heat treatment by means of electron microscopy and X-ray diffraction analysis manifested no crystallinity in the skin glass but a significant crystal content was observed in the core glass. Two distinct phases were indentified in the core glass, viz., CaF.sub.2 and crystals having a xonotlite structure. Xonotlite is a fibrous hydrated silicate of calcium whose chemical composition Ca.sub.6 Si.sub.6 O.sub.17 (OH).sub.2 is closely related to that of wollastonite (CaSiO.sub.3). In the opal core glass the hydroxyl group is most likely replaced by fluorine which has the same charge and approximately the same ionic radius. The composite core body having an overall coefficient of thermal expansion less than that of the crystal-free glass. This lowering of expansion coefficient leads to a decreased differential existing between that of the skin glass and the core glass with consequent reduction in surface compression and loss of mechanical strength in the laminated body.
Therefore, the principal objective of the present invention is to limit development of crystallization in the core glass, especially the low expansion crystals having a xonotlite structure, without deleteriously affecting the other properties of the glass when the laminate is heated to temperatures above the annealing point thereof but below the softening point.